This invention relates generally to the field of anechoic chambers. More specifically, this invention relates to a dual-chambered anechoic chamber used in conjunction with spatial averaging for making transmission measurements of electromagnetic devices.
Large anechoic chambers are used to perform transmission measurements. Building space requirements contribute to the high construction costs of these large chambers, limiting their availability. Box anechoic chambers, which are limited by the box shape and the shape of the aperture, are a less expensive alternative to the large chambers. Enlarging the chamber and thickening the absorber material can achieve increased performance of box chambers, but cost remains a factor.
Thus, there is a need for a relatively small anechoic chamber, which provides accurate transmission measurements without the high costs associated with larger anechoic chambers.
The invention is a dual-chambered anechoic chamber for making transmission measurements of electromagnetic devices. RF transmission characteristics measured include frequency and time responses. Electromagnetic devices, which may be measured using the anechoic chamber of this invention, include any device with a planar structure.
The anechoic chamber includes two tapered chambers: a first chamber with a first aperture and a second chamber with a second aperture opposed to the first aperture. An alignment apparatus aligns the two chambers and positions a test device between the first and second apertures. A transmitter antenna in the first chamber transmits a test signal, which is received by a receiver antenna in the second chamber. Coupled to the first chamber, the anechoic chamber has a positioning mechanism that includes a support means for mounting the transmitter antenna and a control means for changing the position of the transmitter antenna. In a preferred embodiment, the support means includes an extender bar and the control means includes a sliding mechanism and a stepper motor. The extender bar, on which the transmitter antenna is mounted, keeps the sliding mechanism and motor out of the measurement field. The sliding mechanism and stepper motor control the movements of the transmitter antenna in the first chamber.
RF energy returning from nearby reflectors interferes with measurement accuracy. Anechoic chambers eliminate such reflections. When using the dual-chambered anechoic chamber for making measurements, spatial averaging may be used to provide more accurate transmission measurements. At each selected test frequency, measurement data are recorded a predetermined number of times at different transmitter antenna positions. For each transmitter antenna position, two measurements are made: one measurement with the test device positioned between the first and second apertures, and another measurement without the test device. Finally, when all desired transmission measurements have been completed, the measurement data are spatially averaged; i.e., (a) for each transmitter antenna position, the two measurements are scaled by dividing the measurement with the test device by the measurement without the test device; (b) the scaled measurements for the different antenna positions are summed; and (c) the summed values are averaged by the number of antenna positions used. Thus, at the selected test frequencies, each spatially averaged measurement corresponds to several measurements that have been scaled, summed, and averaged, providing a more accurate transmission measurement.